ABSTRACT Mobile App Promoting Sexual Health for Young Black Men who Have Sex with Men Men who have sex with men, and particularly Black men who have sex with men (BMSM) have been disproportionately affected by HIV/AIDS since the beginning of the epidemic, but in recent years the impact of HIV among young Black men who have sex with men (YBMSM) has escalated rapidly. More new HIV infections occur among YBMSM (aged 13-24) than any other age and racial group of MSM, with an increase of with an increase of 87% from 2005-2014. The few interventions focused solely on YBMSM, target ages 18-29, leaving a dearth of interventions for teen BMSM. Further, research has found that sex education for YMSM is not readily available from friends, family, or schools, with many YMSM relying on older partners, the Internet, and pornography for education. This project will address these gaps and fill an important product niche for community-based organizations, health departments, clinics, and schools serving YBMSM by completing the development of a sex positive mobile app delivered HIV/STI intervention developed for use by YBMSM ages 14-17. The program employs a theory based approach to address essential knowledge, perceptions of risk, peer norms, attitudes and skill with two primary goals: (1) To reduce HIV/STI the program emphasizes partner reduction, avoidance of concurrent partners, condoms and HIV/STI tests; and (2) To improve sexual health and relationships, it helps participants to become clearer about what they do/don't want to do sexually, to communicate their choices, and to learn ways to enhance sexual experience while reducing HIV/STI risk. The project offers a number of innovations, including ?the creation of the first sex-positive multimedia interactive smartphone app-delivered HIV/STI prevention program specifically designed for YBMSM aged 14-17 that addresses risk reduction and sexual response, a group difficult to reach via in-person intervention; ?the inclusion of content designed to enhance sexual experience, an approach called for by the target audience; ?a review of the pros/cons of PREP and PEP; ?content tailoring based on user activity and inputs ?context sensitive delivery of booster content, such as monitoring SMS texts for keywords (i.e. hookup, HIV test)--if detected the app responds with context-appropriate content in the moment. Phase I of this project created a prototype app with activities focused on understanding sexual pleasure, communicating online, condom use including fit, and information about lube. A usability evaluation with 24 members of the target audience found the app to be usable and well-liked, and it improved knowledge and attitudes, demonstrating the utility of the app. This Phase II project will refine the prototype based on usability testing results, complete the prototype, adding more than 20 additional content-based activities, peer stories, instant games, and an optional user messaging/forum system. The completed app delivered program will be evaluated using a cluster randomized controlled trial to assess effectiveness in changing sexual risk behaviors and relationship and communication behaviors and attitudes.